In the past, the common method of reading a document sheet with images on both sides was to read one side first, then turn over the sheet and read the back side.
That is, document sheets are loaded at the starting position and sent out to a document reading table one after another. And after one side of the document sheet is read on the document reading table, the document sheet is turned over and brought back to a discharge point. And the reverse side of the document faces the document-reading table and the reverse side is read.
In recent years, a new apparatus has been proposed which simultaneously reads both sides of the sheet instead of reading one side and then another side as in the conventional technique. In an apparatus disclosed in Japanese laid open patent application No. 11-27444, for example, two document-reading units are provided so that as the document sheet is conveyed, the two sides of the sheet can be read at the same time. The mechanism of the apparatus will be explained with reference to FIG. 15.
First, there is provided a document glass 100h on the upper surface of a reduction optical system that performs sub-scanning by moving a light source and mirrors. And a document pressing plate unit 101 is provided over that. On one side of the document pressing plate unit 101 is provided a sheet feeder unit 103.
The sheet feeder unit 103 is provided with a document stacker 108, a preliminary feeding unit 111, a separation unit 110, a document feeding unit 104, a U turn path 103a, an opening A2 defined on the lower surface of the conveying path, a contacted image sensor 113 provided on the upper side of the conveying path and a document discharge unit 105a provided most downstream. Those components are arranged in that order from the document feeding side. The contacted image sensor 113 is an image sensor so designed that a sensor with a short focal distance and its nearby light source are brought into contact with the document sheet to read the surface of the document. Since the contacted image sensor 113 is easy to reduce in size, it is widely used in the sheet-through-type reading apparatuses.
Furthermore, under the opening A2, there is provided an empty space to accommodate a mirror 100a making up an optical system of a book scanner unit 100. And it is so arranged that a light source 100g of the book scanner unit 100 sheds light on the document through the opening A2 and that the document surface thus irradiated is reflected on the mirror 100a accommodated in the space.
This apparatus is provided with the so-called “book scanner unit” in which the document sheet is placed on the document glass 100h and scanned by moving the optical system and a sheet scanner unit 120 having a document conveying apparatus equipped with a sheet separation mechanism in an upper part of the document glass 100h. That permits reading the document sheet by the sheet scanner unit 120 while moving the document sheet unlike the typical conventional copying machine in which the document sheet is conveyed to a specific position to stand still and scanned.
The book scanner unit 100 is provided with a scanning mechanism for scanning the document sheet standing still on the document glass 100h and a charge coupled device (CCD) sensor 100e for photoelectric conversion of a document image inputted through this scanning mechanism. That is, the document sheet placed on the document glass 100h is irradiated with the light source 100g that moves along with the mirror 100a and the reflector 100f to reflect the light from the light source 100g so that the lightness on the document surface reaches a specific level. Then, the document image displayed on the mirror 100a is reflected on a second mirror 100b, a third mirror 100c etc. and then sent to CCD sensor 100e through a group of lens 100d for photoelectric conversion. Then, the electric signal photoelectrically converted by CCD sensor 100e is sent out to an image processing unit (not shown).
The first mirror 100a moves from the first reading position B1 to the second reading position B2 when a still document sheet is read. Meanwhile, when the reverse side of a sheet document S loaded on the document stacker 108 is read, the first mirror 100a moves and stops under the opening A2, and, through the opening A2, reads the document sheet S passing through the document conveying path.
From a pile of the loaded document sheets S, the separation unit 110 separate sheets one by one at the bottom of the pile. Through the document conveying path 103, the document sheet S thus separated is conveyed to the opening A2 and the document face reading unit A3 provided downstream. And while the document sheet S is being conveyed, one side of document sheet S is read by the contacted image sensor 113 in the document face reading unit A3.
The other side of the document sheet S is reflected on the first mirror 100a moving just under the opening A2 and the document image enters CCD sensor 100e through the second mirror 100b, the third mirror 100c etc. and photoelectrically converted by this CCD sensor 100e. The document sheet S thus read is discharged on a paper discharge tray 106 by a document discharge unit 105a. 
FIG. 16 is a block diagram of the overall control system of this apparatus. The configuration of the apparatus will be explained.
First, the outer interface 309 conforms to serial interface standards such as RS 232. That makes it possible to connect the present apparatus to peripheral equipment of the computer. That is, it is possible to utilize the present apparatus as image scanner or local printer, and also network printer.
Also, to the inner interface, the following are connected: scanning unit 302, displaying unit 303, reading unit 304, recording unit 305 and communication control unit 306. The reading unit 304 is made up of a fixing and reading unit 304a and a movement fixing and reading unit 304b. Furthermore, the fixing and reading unit 304a has a sheet scanner control unit 328 and a book scanner control unit 324.
Here, an image synthesizing unit 340 that corresponds to an image synthesizing unit selects and synthesizes image data read by the book scanner unit 100 and the contacted image sensor 113. That is, this apparatus is provided with a selection unit for selecting the image on the moving document (document sheet S) and the image of the still image (book document) page by page. And the image data on the page selected by the selection unit are synthesized by the image synthesizing unit.
Under the arrangements just described, both sides of the document sheet can be read simultaneously and images can be formed one after another. This arrangement for reading both sides at the same time has an advantage that because the document sheet is read as it passes through the apparatus only once instead of reading one side and turning over the page and then reading the other side, the possibility of damaging the document sheet will substantially decrease.
The document reading apparatus just described is so designed as to read both sides of the document sheet at the same time, and therefore, the amount of image data processed per unit time will double. Therefore, the document reading speed has to be halved or two image processing systems have to be provided.
That is, the problem is that if the document reading speed is halved, the productivity will decrease, and if two image processing systems are provided, the cost will increase.